Forbidden Knowledge We investigate the castle that helped create Great Britain and learn why it was such an impregnable fortress. Then it’s off to discover the invisible underground defensive systems of Cappadocia and how they proved so successful they are they longest continually inhabited fortress in history. We test the ancient Mayan killer bee castle defences, and release bees in an attack. We build a replica section of the ancient Roman fort of Alesia and discover whether the great general Julius Caesar could really have built 24 miles of defences in just 6 weeks. We investigate the most impregnable walls in history – the multi-layered defences of Constantinople – and discover the type of cannon that the sultan Mehmet used to finally destroy them in the great siege of 1453. Finally, we fire one of these cannons for the first time in hundreds of years and collect invaluable ballistic data.